The present invention relates to cord, cord reinforced plies and the radial tires for vehicles incorporating them. Radial tires are those tires wherein the cords of the carcass plies which extend from one bead to the other lie substantially on radial planes.
Particularly, the present invention relates to a cord reinforced composite having rubber where the structure is for tires and more particularly to a tire carcass or belt wherein at least one of two plies in the belt has the cords therein biased with respect to the direction of rotation of the tire.
Reinforced elastomeric articles are well known in the art for example for conveyor or like type belts, tires, etc. with cords of textile and/or fine steel wire, particularly belts for pneumatic tires with up to four layers with the cord reinforcement between adjacent layers being opposingly biased with respect to the direction of movement of the tire where it is desired to reinforce in the lateral direction in addition to the direction of rotation of the tire. Further, cords made of multi twisted filaments of fine wire with two or more filaments in a single strand construction having a wrap filament therearound to reinforce the above structure have also been known. More recently multi-strand cords such as 2+7.times.0.22+1 have been found necessary to meet the higher demand of fatigue life for composites in tire belts but are more expensive to make. Even more recently, there has been use of single strand cords of multi-filaments which are not twisted about each other but rather twisted altogether as a bundle or bunch to simplify the cord construction over multi-directional cords. Higher fatigue life requirements for composites in tires have resulted in cords with smaller filament diameter requiring more filaments in the cord to obtain the necessary strength.
Most recently two ply tire belts for light truck tires have been used having cords of 2+2.times.0.03 HT. An example of 2+2.times.0.30 HT cord is given in Assignee's prior U.S. Pat. No. 5,188,685 issued Feb. 23, 1993. These cords were made of high tensile (HT) steel of a carbon content by weight greater than 0.80% which was of a lesser strength than the above steel alloys which will be referred to herein as super tensile (ST).
Many problems have had to be overcome even after development of the above steel alloys and filaments. The higher strength steel alloys resulted in changes in cord modulus giving rise to the possibility of adjusting the parameters of a tire belt gross load which depend upon three factors assuming adequate cord to rubber adhesion. The factors are cord modulus, the ratio of cord volume to rubber volume which is often expressed as the number of cord ends per inch, and the angle of cord reinforcement. As further previously noted, as the angle of cord reinforcement approaches the direction of rotation of the tire the support from the reinforcement in the lateral direction moves toward zero. An increase in the above-mentioned two other cord related factors generally results in an increase of weight for the belt. Added weight means added cost and higher rolling resistance of a tire. Lighter cords with a lower modulus do not solve the problem because even though they have lower weight they also have a lower cord modulus which must be offset by increasing the ratio of cord to rubber volume. This increase in cord volume is limited by the physical size of the cord and the resulting spacing between the cords which governs the ability of the rubber to penetrate between the cords for good cord to rubber adhesion.
A challenge was to determine cord structure which could take advantage of the new cord modulus while not adversely affecting cord volume to rubber volume ratio on lateral reinforcement.
After considerable study, effort, testing and time, the present invention provided cords for truck tire Load Ranges E, F, G, H and J which substantially reduced the number of filaments for these Load Ranges. While a reduction in the number of filaments would lead one to expect a reduction in weight, this would not necessarily be the case where the filament size was increased. Under such circumstances, cord was found for use in the Load Ranges by varying the ends per inch (EPI) in the layers of the belt. More particularly, a single type of cord was found for use across the Load Ranges F-J. Other advantages which exist in the present invention include improved rolling resistance in at least one instance and a reduction in the cord gum coat gauge between the cord layers in the belt in another instance. A weight reduction due to reduction in weight of reinforcement as well as reduction in an amount of gum gauge also result in a reduction in cost for a tire of the present invention. Further, the new belt structure gives better rolling resistance perhaps because of the higher stiffness of the new cord over the old cord being used for reinforcement in the belt structure.